AAS 203rd Meeting, January 2004
Session 132 Gamma Ray Bursts: Processes and Phenomena
Oral, Thursday, January 8, 2004, 2:00-3:30pm, Centennial IV

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[132.04] Numerical Simulations of Relativistic Jets in Collapsars

W. Zhang, S. E. Woosley (UCSC), A. Heger (LANL), A. I. MacFadyen (CalTech)

The propagation and break out of relativistic jets in collapsars, which are believed to give rise to outburst of high-energy emission known as gamma-ray bursts (GRBs), are examined in multi-dimensional numerical simulations using a special relativistic hydrodynamics code. If powered long enough, a relativistic jet from a collapsar can break out of a massive Wolf-Rayet star. During its propagation, the jet is collimated by the passage through the stellar mantle. Starting with an initial half-angle of up to 20 degrees, it emerges with an half-angle that, though variable with time, is around 5 degrees. Interaction of the jet with the star and its own cocoon also causes mixing that sporadically decelerates the flow. As it erupts, the highly relativistic jet core (3 to 5 degrees) is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta (\Gamma ~15) that expands and becomes visible at larger polar angles (~ 10 degrees). We predict a distribution of energy and Lorentz factor with viewing angle in the jet beam and its cocoon. These results have important implications for the observed light curves and energies of GRBs and imply that what is seen may vary greatly with viewing angle. In particular, we predict the existence of a large number of low energy GRBs with mild Lorentz factors that may be related to GRB 980425/SN 1998bw and to the recently recognized XRFs. Jet stability is also examined in three-dimensional calculations. It is found that a three-dimensional jet undergoes a kink instability. Processing jets are also examined in three-dimensional simulations. If the jet changes angle by more than three degrees in several seconds, it will dissipate, producing a broad beam with inadequate Lorentz factor to make a common gamma-ray burst.

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.